Thermal imaging is a well known type of imaging in which a heat sensitive medium is subjected to radiation, such as a laser beam, to raise the temperature of the medium above a threshold temperature at which the properties of the medium are modified to record, either positively or negatively, an image on the medium. The thermal sensitive medium may be metallic or non-metallic, e.g., polyester film or aluminum plate material.
Thermal imaging is essentially a threshold energy process. When sufficient energy is applied to the thermally sensitive medium, its properties are transformed such that the image is recorded thereon with or without wet or dry processing. Thermal recording processes can vary from the ablation of metals to the transfer of non-metallic material to change of phase of materials. However, notwithstanding the process being utilized, the energy applied for recording must exceed a threshold recording energy level associated with the medium on which the image is to be recorded.
In optical thermal imaging systems, such as imagesetters and platesetters, a laser diode or solid state laser typically serves as the radiation source. The laser generates and emits a laser light beam which is then scanned on the thermal sensitive medium to record the image. The scan speed and laser power are set such that sufficient energy is applied to the medium to increase the medium temperature to above the threshold recording temperature to record the image. Accordingly, the laser power and scan speed of imaging systems are typically conjunctively determined.
High power lasers are utilized in commercial prepress imaging systems. Using such lasers, the imaging beam can be scanned at a high speed over the medium while still having the laser light beam impinge upon the medium for a sufficient period of time to increase the temperature of the medium beyond the threshold temperature necessary to record the image.
Various techniques have been previously proposed to reduce the radiation source power required to record an image on thermally sensitive medium. For example, it has been proposed to utilize a second radiation source to apply radiation onto an area of the medium to be transformed in order to preheat the medium to some extent before applying a separate radiation beam from a laser light source to record the image on the medium. However, such systems are complex in that they require that the two radiation sources be operated in a coordinated manner. Additionally, the cost of such systems are increased by the expense of the additional radiation source and its installation during fabrication of the system. Because the beams from the respective sources cannot, in practice, be perfectly aligned, a radiation loss occurs and therefore the total energy actually required for recording will exceed the theoretical energy level required for recording. Further still, the additional energy required to operate the second radiation source adds to the operating cost of the system.
Another proposed technique is to preheat a small portion of the area of the medium which will be transformed to a temperature below the threshold recording temperature and thereby modify the optical properties of the medium in a small portion of the area to be transformed, and to then apply the imaging beam over the entire area of the medium to be transformed, including the portion to which the preheating beam was applied, to record the desired image. Because the small portion to which the preheating beam was applied is completely overlapped by the area to which the imaging beam is applied to form the desired image, the small portion of the medium transformed by the preheating radiation beam does not affect the overall dimensions of the larger area of the medium transformed by the imaging beam. This technique may reduce the modulated energy required to form an image to some extent but at the price of increasing the total energy required to image.
Accordingly, a need exists for a lower energy, higher quality, single radiation source per pixel imaging system.